iv INTRODUCTOUY DISCOURSE OF THK 



long beam ] asa through three times the space, it must more with three times the swiftness of the short 

 beam's end, since both more in the same time; and therefore any force applied to the long end must 

 overcome the resistance of three times that force applied at the opposite end, since the two ends move in 

 contrary directions : hence one pound placed at the long end would balance three placed at the short end. 

 The beam we hare been supposing is called a Lever, and the same rule must evidently hold for all propor- 

 tions of the lengths of its arms. If, then, the lever be seventeen feet long, and the pivot, or fulcrum (as it 

 is called, from a Latin word signifying tupport), be a foot from one end, an ounce placed on the other end 

 will balance a pound placed on the near end; and the least additional ueigbt, or the slightest push or 

 pressure on the far end, so loaded, will make the pound weight on the other move upwards. If, instead 

 of an ounce, we place upon the end of the long arm the short arm of a second beam or lever supported by 

 a fulcrum, one foot from it, and then place the long arm of this second lever upon the short arm of a third 

 lever, whose fulcrum is one foot from it ; and if we put on the end of this third lever's long arm an ounce 

 weight, that ounce will move upwards a pound on the second lever's long arm, and this moving upwards 

 will cause the short arm to force downwards sixteen ]*ounds at the long end of the first lever, which will 

 make the short end of the first lever move upwards, though two hundred and fifty-six pounds be laid on 

 it : the same thing continuing, a pound on the long arm of the third lever will move a ton and three- 

 quarters on the short arm of the first lever ; that is, will balance it, so that the slightest pressure with the 

 finger, or a touch from a child's hand, will move as much as two horses can draw. The lever is called, on 

 this account, a mechanical power ; and there are five other mechanical powers, of most of which its properties 

 form the foundation ; indeed they have all been resolved into combinations of levers. The pulley seems 

 the most difficult to reduce under the principle of the lever. Thus the wheel and axle is only a lever moving 

 round an axle, and always retaining the effect gained during every part of the motion, by means of a rope 

 wound round the butt end of the axle ; the spoke of the wheel being the long arm of the lever, and the 

 half diameter of the axle its short arm. By a combination of levers, wheels, pulleys, so great an increase 

 of force is obtained, that, but for the obstruction from friction, and the resistance of the air, there could be 

 no bounds to the effect of the smallest force thus multiplied ; and to this fundamental principle Archimedes, 

 one of the most illustrious mathematicians of ancient times, referred, when he boasted, that if he only 

 had a pivot or fulcrum whereon he might rest his machinery, lie could move the Earth. Upon so simple 

 a truth, assisted by the aid derived from other sources, rests the whole fabric of mechanical power, whether 

 for raising weights, or cleaving rocks, or pumping up rivers from the bowels of the earth ; or, in short, 

 performing any of those works to which human strength, even augmented by the help of the animals whom 

 Providence has subdued to our use, would prove altogether inadequate. 



The application of Dynamics to the pressure and motions of fluids, constitutes a science winch receives 

 different appellations according as the fluids are heavy and liquid like water, or light and invisible like air. 

 In the former case it is called HydroJynamict, from the Greek words signifying water, and power or 

 force; in the latter Pneumatic*, from the Greek word signifying breath or air; and Hydrodynamics is 

 divided into IfyJrottatict, which treats of the weight and pressure of liquids, from the Greek words for 

 lalanciny of water; and J/ydraulict, which treats of their motion, from the Greek name for certain musical 

 instruments played witli water in pipe*. 



The discoveries to which experiments, aided by mathematical reasoning, have led, upon the pressure 

 and motion of fluids, are of the greatest importance, whether we regard their application to practical 

 purposes, or to their use for explaining the appearances in nature, or their singularity as the subjects of 

 scientific contemplation. When it is found that the pressure of water or any other liquid upon the surface 

 that contains it, is not in the least degree proportioned to its bulk, but only to the height at which it 

 stands, so that a long small pipe, containing a pound or two of the fluid, will give the pressure of twenty 

 or thirty tons ; nay, of twice or thrice as much, if its length be increased and its bore lessened, without tho 

 least regard to the quantity of the liquid, we are not only astonished at so extraordinary and unexpected a 

 property of matter, but we straightway perceive one of the great agents employed in the vast operations of 

 nature, in which the most trifling means are used to work the mightiest effects. We likewise learn to 

 guard against many serious mischiefs in our own works, and to apply, safely and usefully, a power calculated, 

 according as it is directed, cither to produce unbounded devastation, or to render tho most beneficial 

 service 



Mor are tho discoveries relating to the Air less interesting in themselves, and less applicable to 

 important uses. It is an agent, though invisible, as powerful as Water, in the operations both of nature 

 and of art Experiments of a simple and decisive nature show the amount of its pressure to be between 

 14 and 15 pounds on every square inch ; but, like all other fluids, it presses equally iu every direction : 

 so that though, on the hand, there is a pressure downwards of above 250 pounds, yet this is exactly 

 balanced by an equal pressure upwards, from the air pressing round and getting below. If, however, the air 

 on one side be removed, the whole pressure from the other acts unbalanced. Hence the ascent of water 

 in pumps, which suck out tho air from a barrel, and allow the pressure upon the water to force it up 32 or 

 33 feet, that body of water being equal to the weight of tho atmosphere. Hence the ascent of the mercury 



